Alizarin is a vegetable dye produced from the root of Madder, also commonly referred to as Rubia. It is used most for dying wool, cotton and other fabrics as well as leather. It can be used to produce red, yellow and brown color.
Alizarin is found in only one corner of the Shroud and not anywhere else on the cloth. This corner is where the carbon dating sample was removed. The fact that it is found there and nowhere else forms part of the argument that the carbon dating sample was taken from a repair to the cloth.
The alizarin dye was complexed with a common mordant, alum (hydrous aluminum oxide). This is a common mordant used in dying fabric.
The carbon dating samples also contain cotton fibers and spliced threads where apparently newer thread was dyed to match age-yellowed older thread.
In addition to alizarin, other dyestuffs, cotton fibers and spliced threads are NOT found elsewhere on the Shroud. As Raymond Rogers wrote in the peer-reviewed, scientific journal Thermochimica Acta:
The combined evidence from chemical kinetics, analytical chemistry, cotton content, and pyrolysis/ms proves that the material from the radiocarbon area of the shroud is significantly different from that of the main cloth. The radiocarbon sample was thus not part of the original cloth and is invalid for determining the age of the shroud.